I have a system running on a Phenom II X4 965 BE proc with 4 gb ram, Radeon HD 6870 running on a 650 watt power supply. I'm thinking of buying a second one so I can run cross-fire but will I be bottle-necking my system by doing that or no? And should my power supply be good enough or would I need something stronger?

What is the model of PSU. 650w should be fine if it's a decent one. I'd consider getting a new mobo and an intel CPU if I was you. Your 6870 shouldn't be having trouble with much right now? Having said that two 6870's should chomp anything you throw at them for a year or two.
Do you have a SSD yet? Consider that also if you don't.

The model of my PSU is here http://www.newegg.co...N82E16817139005 In January of 2013 the computer will be 2 years old because thats when I built it. I didnt have the money back then for an intel build so thats why I went with an amd build. Also, no I don't have an SSD yet but have heard the increase in performance is so much better. Can you recommend a good one for cheap?